1. Field of the Invention
Polymer dispersions on an aqueous basis are characterized by the presence of dispersely distributed polymer particles in an aqueous dispersion fluid. For stabilizing a system of this type in which discrete polymer particles, for thermodynamic reasons, tend to coagulate, as a rule, surface active substances are used. The capability of using starch and the derivatives thereof as surface active substances of this type for generating aqueous polymer dispersions has already repeatedly been suggested.
2. Description of the Related Art
EP 0 276 770 is concerned with a sizing agent in the form of an aqueous polymer dispersion wherein a monomer mix of 20-65% acrylonitrile, 80-35% acrylic acid ester and 0-10% of other ethylenically unsaturated monomers in the aqueous phase is copolymerized in one step, using modified starch of a reduced viscosity of 0.04 to 0.12 dl/g.
EP 0 536 597 A1 describes aqueous polymer dispersions obtainable by radical polymerisation of unsaturated monomers containing at least one reduced starch product obtainable by hydrolysis in the aqueous phase and having an average molecular weight of between 2.500 and 25.000.
In none of the afore-mentioned cases, the way of carrying out the radical polymerisation is suitable for preparing an amphoteric copolymer. The afore-mentioned patent applications do not describe any charge characteristics actually required for these systems in order to judge their suitability for special applications, such as sizing and coating.